SIMOX substrates and bonded SOI wafers are known as primary types of SOI substrates having single crystal silicon layers formed on an insulator such as silicon oxide. SIMOX substrates are SOI substrates obtained by implanting oxygen ions into a single crystal silicon substrate by implantation of oxygen ions, and subsequently performing annealing treatment for chemical reaction between the oxygen ions and silicon atoms to form a buried oxide layer.
Bonded SOI wafers, on the other hand, are SOI substrates obtained by bonding two single crystal silicon wafers one of which is oxidized on the surface thereof but the other is not, and thinning one of the two wafers.
Such SIMOX substrates and bonded SOI wafers employ silicon single crystals grown by the Czochralski (CZ) process or the magnetic-field-applied CZ process. Such silicon single crystals normally contain void defects, which constitute a problem for fabrication of devices using SOI substrates. In the case of SIMOX substrates, void defects are exposed on the surface single crystal silicon layer (SOI layer) after high-temperature heat treatment, being observed as square surface pits. These surface pits are undesirable for device fabrication because they lead to film thickness variation of the SOI layer. Also, some of the surface pits will impair the reliability of a gate oxide film of a MOSFET fabricated using the SIMOX substrates, and therefore an absence of surface pits is desired. In bonded SOI wafers as well, if the thickness of the SOI layer is comparable to the size of the void defects, the void defects in the SOI layer will form pits passing through the SOI layer and, for example, when immersed in a 25 wt % HF solution, the buried oxide layer elutes out to exhibit circular depressions. These are therefore referred to as HF defects, and because such SOI-penetrating pits destroy the very structure of the device, their complete absence is preferred in SOI layers.
The following two methods have been devised in order to avoid generation of such pits. The first is a method using an “epiwafer” with single crystal silicon epitaxially grown on a single crystal silicon wafer. This method is used because void defects that can lead to pits are not present in the epilayer of epiwafers. The other method is a method using a nitrogen-added single crystal silicon wafer during the crystal growth in the CZ process, as proposed in Japanese Unexamined Patent Publication HEI No. 10-64837. By controlling the nitrogen concentration it is possible to reduce or eliminate pits. However, the epiwafer is not a preferred choice, because of the high cost of epiwafers. In nitrogen-added single crystal silicon substrates, a higher nitrogen concentration gives a higher effect of preventing pit generation but tends to result in polycrystallization during crystal growth with increasing nitrogen addition, such that a trade-off exists with respect to productivity and cost.
It is an object of the present invention to solve the problems described above by providing an SOI substrate with no pit generation in the SOI layer, which is producible at low cost and at high productivity, and which has excellent gettering capacity.